The processes of Na, Cl, HCO3 and water transport in turtle bladder epithelium are more directly approachable than those in the kidney for studies on transport mechanism are permeability. The general approach is to determine: how the epithelial cells regulate body fluid composition; how the permeability of and transport mechanisms in each cell membrane regulate the cell composition; and finally how certain transport-related enzymes in the isolated membranes regulate directionality and energy supply of ion transfers. Immediate aims are: (1) to characterize the regulation of ion transfers across each membrane of the bladder cell by the use of pharmacological and physiological agents known to act specifically on conductance channels or pump mechanisms; and (2) to compare the nature of such changes in ion transport parameters (e.g. flux, potential and conductance) of whole tissue with the correspondingly-induced changes in enzymatic parameters (e.g. ATPase, carbonic anhydrase) of isolated membranes from the tissue. Correspondingly, our current data suggest: (1) that Na moves across the mucosal membrane through Na-selective conductance channels which can be specifically blocked by amiloride; that Na is moved actively across the serosal membrane via coupling with a " pump-carrier-ouabain- sensitive-ATPase" system; that Cl and HCO3 are actively transported (independent of Na transport) across the mucosal cell membrane via coupling with an acetazolamide-sensitive carbonic anhydrase; and that histamine stimulates HCO3 transport specifically; and (2) that hydrogen ion and aldosterone produce changes in membrane ATPase reactions which can be correlated with their effects on Na transport; and that histamine, histidine or imidazole, which stimulate purified carbonic anhydrase in vitro, also stimulate HCO3 transport in the intact tissue. Methods for whole tissue include: ion flux, short circuit current, electrical potential, resistance, tissue electrolyte composition, qO2. Methods for cell fractions and microsomal membranes include: transport- related enzyme activities - e.g. ATPase, phosphoprotein formation, ATP binding, ADP:ATP exchange, carbonic anhydrase, and NADH-cytochrome-c- reductase. Electron microscopy of tissue and particulates has been initiated.